This invention relates to automotive tire pressure sensors. More particularly, this invention relates to a battery powered method and system for monitoring internal tire pressure of vehicle tires using a sensor system with improved sensitivity and a power saving device.
Tire pressure sensor systems are known and are commonly used to monitor the internal air pressure in individual pneumatic tires of a vehicle and to provide a warning signal to the driver whenever the internal air pressure in one or more of the vehicle tires is dangerously low or high. The warning signal is typically generated by an r.f. signal generator controlled by a microprocessor connected to the tire pressure sensor, the warning signal being generated whenever the internal tire pressure measured by the sensor lies outside a predetermined normal operating range, signifying either a high or a low pressure condition. This r.f. signal is transmitted to a vehicle-mounted receiver, which uses the warning signal to alert the driver either visually (by activating a warning lamp or display) or audibly (by activating an audible alarm) or both. Electrical power to the sensor circuitry is provided by a battery, which must be replaced when the available power drops below a useful level.
Known tire pressure systems, such as that disclosed in commonly assigned, co-pending patent application Ser. No. 10/346,490 filed Jan. 21, 2003 for “External Mount Tire Pressure Sensor System”, the disclosure of which is hereby incorporated by reference, use a mechanical strain sensor having an essentially linear variable resistance characteristic in one branch of an electrical bridge circuit to measure the internal pressure of a tire to which the sensor is attached. This type of sensor is relatively insensitive to mechanical vibrations, which are regularly encountered in an automotive environment. In addition, the configuration of the electrical circuitry (i.e., the electrical bridge circuit) is relatively simple, has well-known performance characteristics, and has been found to be reasonably reliable in operation.
In spite of the effectiveness of the known sensor circuitry using the strain sensor and bridge circuit, there are inherent limitations which limit the performance of such devices. Firstly, due to the fact that only a single variable resistance element (the strain gauge) is incorporated into one branch of the bridge circuit, the sensitivity of the sensor circuit is limited to the variable resistance range of the single strain gauge used. This limits the potential measurement range of the sensor system. In addition, the known sensor circuitry is susceptible to measurement inaccuracies due to different coefficients of thermal resistivity of the variable resistance strain sensor and the fixed resistances forming the bridge circuit. Secondly, since the sensor circuitry is continuously powered by the essential battery, the useful lifetime of the battery is limited by the battery energy capacity. This drawback is compounded by the need for components having relatively small physical size due to installation constraints. As a consequence, battery replacement is a major constraint to the efficacy of such known sensor systems.
Efforts to provide a simple yet accurate and durable tire pressure monitoring system devoid of the above-noted disadvantages have not been successful to date.